This invention relates to multi-element information transmission systems and more particularly to a multi-element information transmission system suitable for inputting a multi-element information signal string including plural kinds of information signals such as video signals of television cameras, process signals and status signals of control units and transmitting specified one and/or ones of these signals with priority.
Generally, in various plants, transmission of various kinds of signals such as video signals of television cameras, process signals and status signals of control units is needed for the purpose of promoting controlling and monitoring functions of the plants. Of these signals, some signals have a small amount of information but have response characteristics to be evaluated significantly and some have a large amount of information. Accordingly, it is desired to transmit with priority a specified piece of information as necessary. For example, JP-A-56-111345 discloses an information priority transmission system representative of a technique directed to the above transmission system for plants.
According to the system disclosed in this publication, during normal transmission, a transmission frame consisting of a sync word, a station number word and an item word is prepared in a particular station and this frame signal is transmitted to different stations. The item word herein referred to consists of various data items. During priority transmission, the particular station prepares a transmission frame consisting of a sync word, a priority transmission flag, a station number word and a priority transmission word, and this frame signal is transmitted to different stations. The priority transmission flag herein referred to indicates that a specified data is being transmitted with priority. In addition, the particular station selects the data subject to priority transmission from the item word to be transmitted during the normal transmission and reconstructs the priority transmission word by assembling the selected data in a single item word.
Thus, in this known system, the priority transmission word is prepared by the particular station and transmitted with priority during the priority transmission.
Since, in a data transmission system for use in a plant, many kinds of information are transmitted from terminals to a central controller, the respective terminals must be synchronized with each other for transmission of the information. When, in such a transmission system, a specified piece of information is transmitted with priority in the conventional manner, the priority transmission of the specified information piece (intended for shortening the transmission period of the specified information piece) will disturb the synchronization between the respective terminals, resulting in the loss of the specified information to be transmitted. If making an attempt to transmit the necessary information by using the synchronization settled during normal transmission, the information to be transmitted with priority must be added with empty slots so that the number of time slots for the priority transmission may even the number of time slots settled for the normal transmission. This prevents the preparation of a priority transmission word by the use of only the information to be transmitted with priority, thus failing to shorten the transmission period.
One may refer to other known techniques such as a data transmission system disclosed in JP-A-56-87953 and a priority transmission system disclosed in JP-A-56-111346. In these known systems, a particular terminal also prepares an information piece of its own to be transmitted with priority and transmits the information piece, with the result that the aforementioned problems are encountered. Especially, the conventional techniques have unapplicability to the data transmission in plants wherein a great amount of data from a great number of terminals are to be transmitted.